Characterization of two photon excited fragment spectroscopy (TPEFS) for HNO detection in gas-phase kinetic experiments

We have developed and tested two-photon excited fragment spectroscopy (TPEFS) for detecting HNO 3 in pulsed laser photolysis kinetic experiments. Dispersed (220-330 nm) and time-dependent emission at (310 ± 5) nm following the 193 nm excitation of HNO 3 in N 2 , air and He was recorded and analysed to characterise the OH(A 2 Σ) and NO(A 2 Σ + ) electronic excited states involved. The limit of detection for HNO 3 using TPEFS was ∼5 × 10 9 molecule cm −3 (at 60 torr N 2 and 180 μs integration time). Detection of HNO 3 using the emission at (310 ± 5 nm) was orders of magnitude more sensitive than detection of NO and NO 2 , especially in the presence of O 2 which quenches NO(A 2 Σ + ) more efficiently than OH(A 2 Σ). While H 2 O 2 (and possibly HO 2 ) could also be detected by 193 nm TPEFS, the relative sensitivity (compared to HNO 3 ) was very low. The viability of real-time TPEFS detection of HNO 3 using emission at (310 ± 5) nm was demonstrated by monitoring HNO 3 formation in the reaction of OH + NO 2 and deriving the rate coefficient, k 2 . The value of k 2 obtained at 293 K and pressures of 50-200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values. We have characterised two-photon excited fragment spectroscopy (TPEFS) for detecting HNO 3 and demonstrated its application in real-time kinetic studies by monitoring HNO 3 formation in the reaction of OH + NO 2 and deriving the rate coefficient.